NE0 Posted September 21, 2014 Share Posted September 21, 2014 Hi Guys Heres a mystery problem. Past week I've had an unexplained engine problem. Happened twice now!!! Nothings changed recently,( other than new carb needle and jets back in February) but its been ok since then. 1st occurrence. Rode bike home 7miles no problem. Got to the back gate, got off bike with the engine ticking over and used the headlight to unlock the gate. Whilst I was off the bike and by the gate the engine started to race away, nobody was on it!!! it was if some one had pulled open the throttle! My thoughts instantly was perhaps the steering whilst parked was somehow affecting the cable,, quickly returning to the bike and setting the bike straight the bike continued to rev faster and faster, I switched the engine kill, and it seemed to continue!!!! pulled out the keys, no effect! PANIC!!!! Then put it in gear, braked hard and stalled it!........weird or what? Same things just happened!! 11:00pm at night with my bike racing and nobody on it!! but it was ticking over when i got off and walked away and the steering was straight ahead..... I was 8 ft away when all of a sudden the engine started racing away. very rapidly going from tick over to screaming!!! same scenario as above. What the F.......is going on? .........................................................................Poltergeist? Link to comment Share on other sites More sharing options...
wild foamy Posted September 22, 2014 Share Posted September 22, 2014 Carb floats...? Link to comment Share on other sites More sharing options...
KevtheRev Posted September 22, 2014 Share Posted September 22, 2014 Ah yeah , your bike is definitely haunted . Seriously though , check the cable routing ,(you've probably done this already ) . Then check all the gaskets and rubbers between the airbox and cylinder inlet , as it sounds to me as if the bike is scavenging air from somewhere causing the engine to race . If it was 40 yrs ago I'd have advised you to check the inside of the cylinder head for carbon deposits which heat up and cause a condition called "pre ignition" , where the red hot carbon fires the fuel charge prematurely . I haven't seen that one in a long time due to modern fuels and two stroke oils being cleaner . Also check your ignition switch wiring as the bike should've stopped when you switched it off . Best of luck ! Link to comment Share on other sites More sharing options...
Moderator Cynic Posted September 22, 2014 Moderator Share Posted September 22, 2014 On the sidestand the carb floats dont move so the carb runs dry. If it stops just right a little fuel will sill be getting in to keep it running. But as it gets leaner it gets faster. If its lean enough, in extreem it will auto ignite. mine has done it. Caused by a suspention lift and leaning further on the stand. Scary but nothing wrong. 1 Link to comment Share on other sites More sharing options...
KevtheRev Posted September 22, 2014 Share Posted September 22, 2014 That makes a lot of sense Cynic . I never ran into that problem with any of my DT's , thankfully . Link to comment Share on other sites More sharing options...
NE0 Posted September 22, 2014 Author Share Posted September 22, 2014 Thanks for your replies so far... I've been also 'googling' theres a chap on the ATM website who describes the SAME problem, but as usual doesn't come back to say how it was fixed http://www.allthingsmoto.com/forums/f-30/dt175-poor-idle-run-away-reving-help-43078/ The answer at the end about a new choke is identical to the same reply on another DT175 post the poster has the same name. Probably works for the company which sells chokes! It's happened again today!! but I was sitting on the bike this time, ie I didn't get off and put it on the side stand! Looks like I need to get my spanners out!!!!!!!!!!!.................... Link to comment Share on other sites More sharing options...
Moderator Cynic Posted September 22, 2014 Moderator Share Posted September 22, 2014 Ah. That changes things. Will still be air though. What are the carb joints like. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted September 22, 2014 Moderator Share Posted September 22, 2014 blocked filters? check you flow rate NEO Link to comment Share on other sites More sharing options...
NE0 Posted September 22, 2014 Author Share Posted September 22, 2014 Been busy 'googling' this problem, eventually found what appears to be the problem.........anyone heard of LEAN RUNAWAY? Its often related to AIRLEAKS just as cynic suggested. According to various websites:- Although not a complete answer in this post it points to leaking seals and inlet rubbers as a possible source of the problem http://www.allthingsmoto.com/forums/f-30/dt175-poor-idle-run-away-reving-help-43078/ other sources altering the mixture/air ratio..... fuel related runaway When you start to run out of gas, your fuel/air mixture can lean out. A lean mixture will increase combustion tempuratures. So basically, when you lean out, your bike will begin to produce more heat. This extra heat creates a hot spot inside the combustion chamber, usually the tip of the spark plug. It gets RED hot, hot enough to ignite the fuel without a spark. Now it's running on it's own, which creates even more heat. More heat means the tip of the spark plug gets even HOTTER, which ignites the fuel even easier, which creates more heat, ect... This is why it is called a runaway, it feeds on itself. Hitting the kill switch or pulling the spark plug wire doesn't fix the problem, because it's not running off the spark anymore. http://www.atvriders.com/vbb/archive/index.php/t-105214.html The solution of stopping the runaway........ not the cure Ok for all you 2 strokers that has had this happen... Do this:- turn off ignition and or kill switch and pin the throttle wide open! The second that you do this the lean condition gets eliminated and the engine will die. This is provided that the bike has proper main jets or on the rich side. Runaway engines happens when the conditions are just perfect with being too lean and the sparkplug becomes a glowplug. When a engine has a proper fuel ratio the fuel will kill the glowplugging and if the ignition is off the engine just shutdowns and dies. Chuck Team Scream Racing http://www.2strokeworld.com/forum/index.php?topic=18755.5;wap2 Link to comment Share on other sites More sharing options...
blackhat250 Posted September 22, 2014 Share Posted September 22, 2014 Yip, a diesel engine can do this also, [ as no spark required] if the oil seal on turbo oil feed passes , the engine oil is sucked thru, and revs it nuts off till there is no oil left in sump. then bang,,, seizure, Link to comment Share on other sites More sharing options...
NE0 Posted September 24, 2014 Author Share Posted September 24, 2014 (edited) Traced the faults......................AIRLEAKS. Blowing or sucking!...... the cylinder head base gasket leaking. and cracked carb inlet rubber joining the carb to cylinder head. Smeared builders silicone around the base gasket and over the carb rubber. Whilst it only lasted one journey and a bit before coming adrift, the engine didn't runaway. However, once the heat took it off when I came home and stopped and got off the bike...........off she went!! Problem isolated at least. Purchased new gaskets and inlet rubber and a set of new reeds (might as well change them) so should be a permanent fix. Its actually probably been leaking for longer than I thought. I've turned into a detective!...... The clue was getting off the bike to unlock the gate. Up until last week my wife has been opening the gate before I got home so i could ride straight in, of course as soon as i was in i killed the engine at low revs. it never did a runaway. It was only as the nights have drawn in I said leave the gate to me. which meant getting off the bike and walking away from it. those 20 seconds or so of tick over and air leak resulted in the runaway. So Holmes thats what happens...said watson! Edited April 6, 2021 by NE0 1 Link to comment Share on other sites More sharing options...
NE0 Posted September 30, 2014 Author Share Posted September 30, 2014 Good news all around..... Stripped the top end down and replaced the base gasket, cleaned the piston and rings (might as well) Installed new reeds and new inlet rubber and then started her up, sounds sweet again! nice tick over and started practically first kick. No lean runaway (although won't be able to confirm that until she's hot again when I go to work tomorrow but no reason to doubt It hasn't been solved) Then I proceeded to install a replacement original exhaust system courtesy of some chap i met at the petrol station. See this post for details And now it sounds amazingly quiet!!...... boy did I have a noisy end can before!! Now its all done... I'm going to have a lay down in the sunshine and top up my tan as I doubt this weather will last much longer! 1 Link to comment Share on other sites More sharing options...
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